Electron guns are employed in a variety of applications in the world today. The components of the electron gun and the method of operation depend upon the particular application. Radio frequency guns are a type of electron gun used as particle sources in certain applications. An RF electron gun uses a time varying electric field to accelerate electrons emitted from a cathode. In a common scenario, an RF electron gun can be employed as a particle source for a free electron laser apparatus designed to generate x-ray radiation. Such x-ray light sources have aggressive electron beam quality requirements, e.g. maximum brightness, lowest transverse emittance beam, and in some cases, sufficient charge in the electron bunches to achieve self-amplified spontaneous emission.
The Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory and the Deutsches Elektronen-Synchrotron (DESY) Free Electron Laser in Hamburg (FLASH) are two test facilities which operate x-ray radiation generating free electron lasers (FEL). Each FEL, in turn, includes an RF electron gun as the initial particle source. One RF electron gun currently employed at FLASH developed at the Photo Injector Test Facility at the DESY's Zeuthen location (PITZ) is a Cs2Te photocathode device accelerated by a 1.5 cell copper cavity operating in the it mode at 1.3 GHz (hereinafter referred to as the PITZ device). Another example of a state-of-the art electron gun currently in use can be found at the LCLS facility. The LCLS electron gun (hereinafter LCLS device) is an RF electron gun comprised of 1.6 cells. Both devices use an electric field to accelerate photo-emitted electrons.
In both cases the RF electron gun design is such that the cathode is disposed in, or immediately adjacent to, the first cell. In these designs, the first cell is always the partial cell. Moreover, the primary design goal for such RF electron guns has historically been to maintain overall electric field flatness in order to achieve a balanced electric field in both the first (partial) cell and second (full) cell. In such cases, the peak on-axis electric field is the same magnitude in both cells.
Alternatively, an unbalanced field is sometimes used in such electron guns, with the peak electric field being of greater magnitude in the second, full cell. Finally, it is also known in the art to marginally increase the field strength in the first cell relative to the second cell in order to reduce the space charge effect. In such cases, the field strength disparity is marginal, at best, and would not vary by more than ten or fifteen percent.
When generating x-ray radiation under such conditions, it is always preferable to have an electron beam of the highest possible quality with the lowest transverse emittance and, consequently, the smallest electron beam spot size. It is therefore an object of this invention to provide an apparatus and method which improves electron beam quality and other beam characteristics in an RF electron gun.